This invention relates generally to touch control systems and, more particularly, to capacitance-responsive touch control input devices for application to horizontal substrates, such as glass ceramic panels. The invention is particularly adapted for use with smooth-top induction, radiant, and halogen burner cooking appliances.
Touch control input devices that respond to the capacitance of a user's contact in order to actuate an appliance are typically applied to a vertical surface. While such orientation is primarily for the convenience of use, it avoids several problems associated with applying touch controls to horizontal surfaces, such as smooth-top cooking appliances. One difficulty with application to horizontal substrates is that there is a greater likelihood that liquids will be splashed on the touch control applied to a horizontal surface, such as a range cook top. Such moisture tends to cause erratic operation of the input control, which could be dangerous in the case of a cooking appliance. This difficulty is typically overcome by separating the touch control from the cooking surface in order to provide a physical barrier between the two. This solution is not without its drawbacks. The primary benefit of a smooth-top cooking appliance is to eliminate the difficulty of cleaning up from spills and boil-over getting into burner elements. While separate touch control input devices are an improvement over electromechanical controls, which still allow places where spills can accumulate, the requirement for a physical barrier between the cook top and the touch control is an impediment to easy cleanup and is a compromise in aesthetic appearance.
An attempt to overcome the problem caused by watery spills on the support surface of a smooth-top cooking appliance causing erroneous operation of a touch control applied directly to the support surface is disclosed in U.S. Pat. No. 4,446,350 issued to Takumi Mizukawa et al. for an INDUCTION HEATING COOKING APPARATUS. In Mizukawa et al., touch pads are provided on the upper surface of a pan supporting plate and are enclosed by guard rings of conductive material composed of a grounded conductor and an enclosing conductor. A control circuit, which is responsive to the touch pads and the guard rings, responds to spilled water or the like contacting the guard rings by latching a power control circuit at a zero power level. This resets the cooking apparatus to a zero heat output condition. The solution proposed in Mizukawa et al. has several difficulties. At least one of the guard rings must be connected with a ground potential in order to be effective. This requires conductive leads being applied to the pan support surface, which is costly and a potential source of failure. Additionally, Mizukawa et al. responds to spilled water by latching the cooking apparatus into a zero output condition. This is a nuisance to the user by requiting that the spill be wiped up and the power level of the cooking apparatus reset in order to continue with the cooking operation.
Another difficulty with applying a touch control to a horizontal substrate is that code requirements, as well as conservative engineering practices, dictate that large horizontal panels be manufactured using particular materials and in a particular manner to avoid breakage due to either mechanical impact or thermal shock. In particular, while relatively thin soda-lime glass may be utilized for vertical touch panels, smooth-top cooking surfaces that are capable of supporting multiple pans above multiple burners are made from glass ceramic material having a greater thickness, on the order of three (3) to five (5) millimeters and require negligible thermal expansion. Additionally, the surface of the substrate facing away from the user is modulated, or dimpled, in order to add greater mechanical strength to the substrate. The thickness of the glass ceramic material and the modulated surface have prevented, in the past, application of touch control technology to such large horizontal substrates.